Thank you for writing.

By putting the plant by a window, you are choosing to not let it go dormant. Nitrogen fertilizer is not needed and may cause exuberant leafing and branching.  Trim as you wish--you will not hurt the plant. 

The plant will not bloom.

Those vines at the top are growing to find light.  They are not finding it so they have no leaves. Trim them off. Weekly haircuts are fine.
Windows can get cold and this may give you some brown leaves.  Cut them off right away.
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/winterizing-mandevillas.htm

Thank you, this is so helpful. Follow-up questions. Would it be better to let the mandevilla go dormant? We're happy with it by the window and enjoy it the way it is, but if it's better for the plant to go dormant, we can do that instead. 

And in the spring, since the arms of the plant will have twisted around the twine by the window, how would we move it back outside? Would we cut the vines off and let it start fresh? 

Thank you again -

Jayson

Thank you for writing back.

In the spring, just gently wrap the plant in something like a drycleaners bag, cut all the strings into as many pieces as needed and then take it out side and rehang it. 
Overall you can trim as much for form now as you wish. For example, make a horizontal lattice or buy bird net and allow the plant to from an interior vine cover for the windows.
You could make it go dormant but green in winter is better.
Best to you.